1. Field of the Invention
The present invention relates to a piezoelectric pressure sensor suited for use in detecting the pressure such as, for example, the combustion pressure inside a cylinder of an internal combustion engine.
2. Description of the Prior Art
Piezoelectric pressure sensors are in wide practical use today and make use of the piezoelectric effect wherein charge is produced by the application of a pressure. In recent years, development of a pressure sensor suited for use in detecting the combustion pressure inside a cylinder of an internal combustion engine has been increasingly pursued. Where a piezoelectric element is utilized for measuring the combustion pressure inside the cylinder of which the internal temperature varies considerably, a base line of the sensor output is caused to drift by the pyroelectric effect of the piezoelectric element. Because this pyroelectric effect is produced in a direction along an axis of polarization of the piezoelectric element, it can be canceled by causing the direction of polarization to be perpendicular to the direction in which a pair of electrodes are opposed.
In view of this fact, a tubular piezoelectric element is used which has an axis of polarization in a direction axially thereof and a pair of electrodes formed on external and internal peripheral surfaces thereof, respectively. Japanese Laid-open Patent Publication (unexamined) No. 3-148028 discloses such a piezoelectric pressure sensor, which is particularly shown in FIG. 1.
The piezoelectric pressure sensor of FIG. 1 has a generally elongated sensor housing 50, a pressure receiving surface 53 disposed at one end thereof, and a mounting screw 51 formed on the external surface thereof. The mounting screw 51 is threaded into, for example, a combustion chamber of an internal combustion engine (not shown) so that the pressure inside the combustion chamber may be appropriately applied to the pressure receiving surface 53. The pressure applied to the pressure receiving surface 53 is then transmitted to a piezoelectric element 55 via a pressure transmission member 54. To this end, the pressure transmission member 54 has one end held in contact with the pressure receiving surface 53 and the other end held in contact with the piezoelectric element 55. The piezoelectric element 55 is pressed against the pressure transmission member 54 by a fixing screw 56 threaded into the sensor housing 50 and is securely held in a prestressed fashion between the pressure transmission member 54 and the fixing screw 56 so that shearing forces may be appropriately applied to the piezoelectric element 55 in proportion to the pressure from the pressure receiving surface 53. The reason for holding the piezoelectric element 55 in the prestressed fashion is to detect not only the positive pressure but also the negative pressure inside the combustion chamber using the piezoelectric element 55. The piezoelectric element 55 has an axis of polarization in a direction axially thereof and also has external and internal electrodes formed on external and internal surfaces thereof, respectively. When the piezoelectric element 55 receives the shearing forces, the charge generated thereby is collected by the external and internal electrodes. The charge collected by the external electrode is extracted by an external charge detection member 58 integrally formed with the fixing screw 56, whereas the charge collected by the internal electrode is extracted by an internal charge detection member (not shown) inserted into and held in the sensor housing 50.
In the pressure sensor of the above-described construction, extraction of the charge from the external electrode of the piezoelectric element 55 is conducted through a metallic contact.
Furthermore, a pressure sensor employing a tubular diaphragm suited for use in detecting, for example, the combustion pressure inside the cylinder of the internal combustion engine has been developed. FIG. 3 depicts a conventional piezoelectric pressure sensor employing such a tubular diaphragm, which sensor is generally used in the internal combustion engine.
The piezoelectric pressure sensor of FIG. 3 comprises a generally elongated sensor housing 75, a piezoelectric element 73 made of crystal and accommodated in the sensor housing 75, upper and lower thermal expansion compensating members 76 between which the piezoelectric element 73 is sandwiched, a pressure transmission base 77 held in contact with the lower thermal expansion compensating member 76, and a pressure receiving portion 71 having a lower surface serving as a pressure receiving surface 71a and an upper surface held in contact with the pressure transmission base 77. The piezoelectric element 73 has external and internal electrodes formed on external and internal surfaces thereof, respectively. The external and internal electrodes are held in contact with a tubular diaphragm 72 and a charge detection member 74, respectively.
The piezoelectric pressure sensor of the above-described construction operates as follows.
The pressure applied to the pressure receiving surface 71a from outside of the sensor housing 75 is transmitted as a compression force to the piezoelectric element 73 via the lower thermal expansion compensating member 76. Because the piezoelectric element 73 has been subjected to a polarization treatment in a direction axially thereof, charge is generated on the electrodes in proportion to the magnitude of the compression force by the so-called longitudinal effect mode (d31) of a piezoelectric phenomenon. The charge generated on the external electrode is led to the sensor housing 75 via the diaphragm 72, whereas that generated on the internal electrode is collected by the charge detection member 74.
The above-described construction is, however, at a disadvantage in that a reliable contact between the diaphragm 72 and the piezoelectric element 73 cannot be obtained at high temperatures due to a difference between the thermal coefficient of expansion of the metallic material and that of the ceramic material. In order to use this pressure sensor in detecting the internal pressure of the combustion chamber of the internal combustion engine, the pressure sensor is required to have temperature characteristics guaranteed in the range of from -40.degree. C. to 250.degree. C. at the location of the piezoelectric element 73 and frequency characteristics guaranteed in the range of from 0.5 kHz to 20 kHz. Under such conditions, problems of contact become large, and in an internal combustion engine attended with severe vibrations, unstable contact causes variations in sensor output.
A method of detecting charge by holding the external electrode with the use of an elastic material is also proposed. FIG. 2 depicts a conventional charge detecting unit employing such a method.
The charge detecting unit of FIG. 2 includes a piezoelectric element 55 and an external charge detection member 58 made of elastic material for radially inwardly biasing the external electrode against the piezoelectric element 55. The external charge detection member 58 detects charge generated at the time a stress is applied to the piezoelectric element 55.
The charge detecting unit of FIG. 2 is, however, at a disadvantage in that the external charge detection member 58 and the external electrode are in point or line contact with each other, thereby enlarging the contact resistance. Furthermore, the elasticity of the external charge detection member 58 deteriorate with age, thus causing an unstable contact.
In addition, when the elastic material is employed, the thickness thereof should be greater than about 1 mm sufficient strength and durability. For this reason, it is practically impossible to make pressure sensors having an outer diameter less than 10 mm.
In general, piezoelectric pressure sensors are intended to detect, for example, the internal pressure of a combustion chamber of an internal combustion engine. When a piezoelectric pressure sensor is mounted on the combustion chamber, a chamber wall of the combustion chamber is required to have an opening defined therein for receiving the pressure sensor. However, because the strength of the engine and combustion gas streams inside the engine are affected by the opening, the outer diameter of the pressure sensor should be made as small as possible.
There is also another problem that the distribution of the pressure applied to the piezoelectric element varies according to assemblage errors and changes the mode ratio of the piezoelectric phenomenon, thereby causing relatively large variations in sensor sensitivity and in temperature-dependent characteristics.
In the construction shown in FIG. 3, when rapid temperature changes have occurred on the pressure receiving surface 71a, for example, in suction strokes or combustion and expansion strokes of the engine, expansion and contraction of the sensor housing 75 caused by heat generated in the proximity of the pressure receiving surface 71a tends to bring about strains in the output waveform. Furthermore, because heat entering the inside of the sensor housing 75 through the pressure receiving surface 71a is directly transmitted to the piezoelectric element 73, the piezoelectric element 73 is subjected to high temperatures and is, hence, deteriorated, resulting in considerable variations in sensor sensitivity. To overcome these problems, a water jacket is generally disposed around the conventional sensor to cool it.
Independently of the expansion and contraction caused by the heat generated in the proximity of the pressure receiving surface 71a, a temperature increase of the sensor housing 75 results in a reduction of the insulating resistance of a signal transmission system and occasionally causes an output drift, or temperature changes of the piezoelectric element change the piezoelectric constant, thereby causing variations in sensor sensitivity. Water-cooling is also required to reduce such influences.